feat: customize variable qualities

[sherwinski]

Description

Related to https://github.com/imgix/js-core#variable-qualities

Add a variable_qualities option to the https://github.com/imgix/imgix-php#fixed-width-images function to allow users to override the mapping of the quality parameter (q) to each device pixel ratio (dpr). This will allow users to have more granular control over the fixed-size srcset values generated.